The present invention is related to minimizing device damage during plasma assisted microfabrication. In particular, it relates to reducing charging damage of dielectrics during microfabrication processing.
The tendency in the art of fabricating integrated circuit (IC) devices is to achieve ever-higher integration (smaller dimensions). During fabrication, various materials are deposited and/or removed in different layers to build the desired integrated circuit. Typically, conductive layers are separated from one another by dielectric materials, e.g., SiO2, and the like. Because semiconductor ICs are fabricated as multilayer structures, there is a common need to interconnect features on one layer with IC features on another. To form such interconnections between IC features, etching through the dielectric materials down to the underlying conducting feature, creates high-aspect ratio channels, typically.
In these instances, low-pressure, high-density plasma typically is used to micro-etch the IC device. When using a plasma in an etch or deposition process, a glow discharge produces a chemically reactive species from a relatively inert molecular gas, which then reacts with the material and either deposits on or etches the material. Any volatile by-products are then removed from the surface. The use of plasma in etching and deposition provides directionality, low temperature and processing convenience.
In such plasma-assisted IC structure patterning, the plasma is often biased with respect to the semiconductor wafer (substrate) to produce desired processing effects. As a result, an excess of positively or negatively charged particles may deposit on the substrate. If this excess deposits on dielectric material, it is likely that the net charge will continue to accumulate and produce an electric field across the dielectric, which can cause damage to the dielectric. Such charging damage may arise due to potential differences between floating gates and the silicon substrate due to plasma non-uniformities. In addition, a layer covered with resist or insulating film with apertures having a high aspect ratio is susceptible to a type of plasma charging damage known as electron-shading damage.
It is believed that electron-shading damage is caused by the difference in behavior between electrons and ions. In general, the semiconductor substrate and the plasma experience a bias potential (electric field) such that the positively charged ions are accelerated towards the substrate, whereas the electrons are decelerated in the electric field. As a result, the velocity of the positive ions becomes very large in the direction towards the substrate and ions are nearly vertically incident to the substrate on contact. In contrast, electrons have much larger angle of incidence to the substrate. This is illustrated in FIG. 1.
Where the conductive layer to be etched has an insulating or dielectric pattern thereon that surrounds the conductive surface, the electrons approaching in an oblique angle are shaded by the dielectric material and can be trapped in the walls of the channel. Ions of normal incidence, i.e., predominantly positive ions, are not shaded by the insulating pattern and are injected into the conductive layer below. This results in an overflow of positive charges into the conductive surface, as illustrated in FIG. 1.
As etching continues, electrons thus captured on the sidewalls of the dielectric layer serve to form an electric field that further repels incoming electrons. In contrast, the positively charged ions are accelerated by the electric field into the channel and onto the conductive surface below. This augments and exacerbates the charge difference between the top and walls of the trench, which can result in increased potential for damage.
Charging damage may arise in numerous ways. One common way occurs when tunneling currents pass through a gate-insulating film in connection with the conductive layer in order to discharge the accumulated charge, as shown in FIG. 1. When the gate-insulating film is thick, the tunneling current is negligible. However, with the higher integration of semiconductor devices, the thickness of the gate oxide films becomes smaller and tunneling current passes more easily and the quality and lifetime of the oxide is degraded.
As a result, dielectric charging can cause permanent damage to the devices being processed. In addition, dielectric charging can result in the degradation of processing properties of the plasma system, such as enhanced notching, trenching and sidewall bowing. Indeed, it has been the consensus among both the scientific and the industrial communities that plasma-process-induced damage is a growing concern in the microfabrication community. This phenomenon is now widely recognized as an important factor limiting yield and device reliability in microfabrication.
The present invention provides a method for the reduction of plasma charge-induced damage in microfabricated devices by reducing the extent of dielectric charging during plasma processing.
In one aspect of the invention, the method includes exposing an article containing a dielectric material that is susceptible to plasma-induced charging to vacuum-ultraviolet (VUV) radiation of an energy greater than the bandgap energy of the dielectric material during or after plasma processing of the article. The temporary increase in conductivity of the dielectric surface due to VUV exposure permits the plasma-induced charge to be conducted from the charging site, or alternatively, prevents charge accumulation in the first instance by allowing charge recombination at the charging site.
In one embodiment of the invention, a method for reducing plasma-induced charging damage in an article includes exposing an article comprising a dielectric material susceptible to plasma-induced charging, to vacuum-ultraviolet (VUV) radiation of an energy greater than the bandgap energy of the dielectric material during or after plasma processing of the article, whereby plasma-induced charging is reduced. The plasma-induced charge is conducted from, or recombined at, the charging site.
In another embodiment, the plasma-induced charge is conducted from the charging site, which may include establishing plasma conditions under which charge is conducted to the plasma or electrically connecting the charging site to ground.
In another embodiment of the invention, the plasma-induced charging is reduced by establishing VUV radiation exposure conditions under which charge recombination takes place at the charging site.
In other embodiments, the article comprises a dielectric material in contact with a conductive surface.
In still other embodiments, a plasma is generated in a plasma processing chamber containing the article, for which the plasma is a source of VUV radiation. In particular, the source of VUV radiation is an argon or oxygen plasma, or a secondary gas is introduced into a processing plasma, and the secondary gas forms a plasma emitting VUV radiation.
In other embodiments of the invention, the step of exposing the article to VUV radiation occurs after a plasma process is complete, or during plasma processing, or alternates with plasma-processing of the article.
In yet other embodiments of the invention, the VUV radiation source provides VUV radiation of an energy and/or flux density sufficient to conduct charge from the charging site, or the plasma has a VUV photon flux of greater than or equal to about 1xc3x971013 photons/cm2-s, or the plasma has a VUV photon flux of greater than or equal to about 1 mW/cm2.
In other embodiments, the VUV radiation is introduced into the plasma chamber separately from the processing plasma, such as by using a glass capillary array.
In another embodiment, a selected portion of the article surface is exposed to VUV radiation, which may be accomplished by masking the surface, or by exposure using glass capillary array of a selected size and shape.
In another aspect of the invention, an apparatus for reducing plasma-induced charging damage in an article is provided, which includes a plasma processing chamber for housing an article; means for generating a plasma; a source of vacuum-ultraviolet (VUV) radiation, in which the VUV radiation is of an energy greater than or equal to the bandgap energy of an article to be plasma processed.
In other embodiments, the apparatus includes conducting means for conducting plasma-induced charge from the article.
In still other embodiments, a plasma is generated in a plasma processing chamber, for which the plasma is a source of VUV radiation, and for example, the source of VUV radiation is an argon or oxygen plasma, or a secondary gas is introduced into the plasma chamber that forms a plasma emitting VUV radiation.
In other embodiments, the plasma is a pulsed plasma. In still other embodiments, the VUV radiation source provides VUV radiation of an energy and/or flux density sufficient to conduct charge from the charging site, and for example, the VUV radiation source is separate from the plasma chamber, or the VUV radiation is introduced from the VUV source into the plasma chamber using a glass capillary array.
The method of VUV-induced charge reduction is applicable to any process in which charge build-up occurs on a dielectric surface. Exemplary processes include metal and oxide plasma-assisted etching and plasma-assisted deposition of oxides, plasma doping, ion implantation, and plasma ashing.